Insertion compounds are those that act as a solid host for the reversible insertion of guest atoms. Cathode materials that will reversibly intercalate lithium have been studied extensively in recent years for use as electrode materials in advanced high energy density batteries and they form the cornerstone of the emerging lithium-ion battery industry. Lithium-ion batteries have the greatest gravimetric (Wh/kg) and volumetric (Wh/L) energy densities of currently available conventional rechargeable systems (i.e., NiCd, NiMH, or lead acid batteries) and represent a preferred rechargeable power source for many consumer electronics applications. Additionally, lithium ion batteries operate around 3.6 volts enabling a single cell to operate in the correct voltage window for many consumer electronic applications.
Lithium ion batteries use two different insertion compounds: for the active cathode and for the anode materials. In a lithium-ion battery, lithium is extracted from the cathode material while lithium is concurrently inserted into the anode on charge of the battery. Lithium atoms travel, or “rock”, from one electrode to the other in the form of ions dissolved in a non-aqueous electrolyte. The associated electrons travel in the circuit external to the battery. Layered rock-salt compounds such as LiCoO2 and LiNiO2(1) are proven cathode materials. Nonetheless, Co and Ni compounds have economic and environmental problems that leave the door open for alternative materials.
LiMn2O4 is a particularly attractive cathode material candidate because manganese is environmentally benign and significantly cheaper than cobalt and/or nickel. LiMn2O4 refers to a stoichiometric lithium manganese oxide with a spinel crystal structure. A spinel LiMn2O4 intercalation cathode is the subject of intense development work (2), although it is not without faults. The specific capacity obtained (120 mAh/g) is 15-30% lower than Li(Co,Ni)O2 cathodes, and unmodified LiMn2O4 exhibits an unacceptably high capacity fade. Several researchers have stabilized this spinel by doping with metal or alkali cations (3,4). While the dopants successfully ameliorated the capacity decline, the initial reversible capacity is no better than 115 mAh/g, and the running voltage of the cell is no better than the usual 3.5 V.
Recently, olivine-structured LiMPO4 where M=Fe, Mn, Co, Cu, V have been gaining interest as candidate materials for rechargeable lithium batteries (5,6 & Goodenough patent). They have a theoretical capacity of up to 170 mAh/g, which would increase the energy density compared to LiCoO2 or LiMn2O4.                In particular Lithium iron phosphate (LiFePO4) has established its position as a potential next generation cathode material. LiFePO4 has advantages in terms of material cost, chemical stability and safety. However, the Fe3+/Fe2+ couple in LiFePO4 has a significantly lower voltage (3.45V versus Li/Li+) when compared to the (4.05 V versus Li/Li+) in the standard LiCoO2 based lithium ion batteries and this lowers the energy available for the LiFePO4 system. In addition LiFePO4 has low electronic conductivity which leads to initial capacity loss and poor rate capability associated with diffusion-controlled kinetics of the electrochemical process. Morphological modification at the nano-scale level appears to be the best tool to control these undesired phenomena.        
The use of olivine type LiMnPO4 would also be of interest because of the position of the Me3+/Mn2+ couple which creates a potential of 4.05V versus Li/Li+ which is compatible with the present LiCoO2 based lithium ion batteries. However LiMnPO4 is an insulator with ca. 2 eV spin exchange band gap and this significantly lowers the electrochemical activity compared to LiFePO4 which is a semiconductor with ca. 0.3 eV crystal field band gap. Furthermore the two-phase Mn3+/Mn2+ redox character also prohibits the introduction of mobile electrons or holes into the band.
Sol-gel processing can control the structure of the material on a nanometer scale from the earliest stages of processing. This technique of material synthesis is based on some organometallic precursors, and the gels may form by network growth from an array of discrete particles or by formation of an interconnected 3-D network by the simultaneous hydrolysis and polycondensation of organometallic precursors.
Based on thermodynamics and kinetics that govern the precipitation of pure phosphate phases. Delacourt et al. (6, 8) described a low-temperature preparation of optimised phosphates.
Dominko et al. synthesized micro-sized porous LiMnPO4/C composite (where M stands for Fe and/or Mn) using a sol-gel technique (9). However, the materials obtained via these “chimie douce” methods, gave disappointing electrochemical performances −70 mAh/g at C/20 are the maximum obtained.
The origin of this poor performance is ascribed to both slow Li diffusion in the solid phase and a poor electronic and/or ionic conductivity of the material. (Delacourt, C; Laffont, L; Bouchet, R; Wurm, C; Leriche, J B; Morcrette, M; Tarascon, J M: Masquelier, C. Journal of the electrochemical society (2005), 152 (5): A913-A921)
A novel approach is required that addresses these issues concurrently, if higher performances are to be achieved.